The Ras proteins (Ha-Ras, Ki4a-Ras, Ki4b-Ras and N-Ras) are part of a signalling pathway that links cell surface growth factor receptors to nuclear signals initiating cellular proliferation. Biological and biochemical studies of Ras action indicate that Ras functions like a G-regulatory protein. In the inactive state, Ras is bound to GDP. Upon growth factor receptor activation Ras is induced to exchange GDP for GTP and undergoes a conformational change. The GTP-bound form of Ras propagates the growth stimulatory signal until the signal is terminated by the intrinsic GTPase activity of Ras, which returns the protein to its inactive GDP bound form (D. R. Lowy and D. M. Willumsen, Ann. Rev. Biochem. 62:851-891 (1993)). Mutated ras genes (Ha-ras, Ki4a-ras, Ki4b-ras and N-ras) are found in many human cancers, including colorectal carcinoma, exocrine pancreatic carcinoma, and myeloid leukemias. The protein products of these genes are defective in their GTPase activity and constitutively transmit a growth stimulatory signal.
Ras must be localized to the plasma membrane for both normal and oncogenic functions. At least 3 post-translational modifications are involved with Ras membrane localization, and all modifications occur at the C-terminus of Ras. The Ras C-terminus contains a sequence motif termed a "CAAX" or "Cys-Aaa.sup.1 -Aaa.sup.2 -Xaa" box (Cys is cysteine, Aaa is an aliphatic amino acid, the Xaa is any amino acid) (Willumsen et al., Nature 310:583-586 (1984)). Depending on the specific sequence, this motif serves as a signal sequence for the enzymes prenyl-protein transferase or geranylgeranyl-protein transferase, which catalyze the alkylation of the cysteine residue of the CAAX motif with a C.sub.15 or C.sub.20 isoprenoid, respectively. (S. Clarke., Ann. Rev. Biochem. 61:355-386 (1992); W. R. Schafer and J. Rine, Ann. Rev. Genetics 30:209-237 (1992)). The term prenyl-protein transferase may be used to refer generally to farnesyl-protein transferase and geranylgeranyl-protein transferase type I. The Ras protein is one of several proteins that are known to undergo post-translational prenylation. Other prenylated proteins include the Ras-related GTP-binding proteins such as Rho, fingal mating factors, the nuclear lamins, and the gamma subunit of transducin. James, et al., J. Biol. Chem. 269, 14182 (1994), have identified a peroxisome associated protein Pxf which is also prenylated. These same authors have also suggested that there are prenylated proteins of unknown structure and function in addition to those listed above.
Inhibition of prenyl-protein transferase has been shown to block the growth of Ras-transformed cells in soft agar and to modify other aspects of their transformed phenotype. It has also been demonstrated that certain inhibitors of prenyl-protein transferase selectively block the processing of the Ras oncoprotein intracellularly (N. E. Kohl et al., Science, 260:1934-1937 (1993) and G. L. James et al., Science, 260:1937-1942 (1993)). Recently, it has been shown that an inhibitor of a prenyl-protein transferase blocks the growth of ras-dependent tumors in nude mice (N. E. Kohl et al., Proc. Natl. Acad. Sci U.S.A., 91:9141-9145 (1994)) and induces regression of mammary and salivary carcinomas in ras transgenic mice (N. E. Kohl et al., Nature Medicine, 1:792-797 (1995)).
Indirect inhibition of prenyl-protein transferase in vivo has been demonstrated with lovastatin (Merck & Co., Rahway, N.J.) and compactin (Hancock et al., ibid; Casey et al., ibid; Schafer et al., Science 245:379 (1989)). These drugs inhibit HMG-CoA reductase, the rate limiting enzyme for the production of polyisoprenoids including prenyl pyrophosphates. Prenyl-protein transferases utilizes prenyl pyrophosphates to covalently modify the Cys thiol group of the Ras CAAX box with a prenyl group (Reiss et al., Cell, 62:81-88 (1990); Schaber et al., J. Biol. Chem., 265:14701-14704 (1990); Schafer et al., Science, 249:1133-1139 (1990); Manne et al., Proc. Natl. Acad. Sci USA, 87:7541-7545 (1990)). Inhibition of prenyl pyrophosphate biosynthesis by inhibiting HMG-CoA reductase blocks Ras membrane localization in cultured cells. However, direct inhibition of prenyl-protein transferases would be more specific and attended by fewer side effects than would occur with the required dose of a general inhibitor of isoprene biosynthesis.
Inhibitors of farnesyl-protein transferase (FPTase), a type of prenyl-protein transferase, have been described in two general classes. The first are analogs of farnesyl diphosphate (FPP), while the second class of inhibitors is related to the protein substrates (e.g., Ras) for the enzyme. The peptide derived inhibitors that have been described are generally cysteine containing molecules that are related to the CAAX motif that is the signal for protein farnesylation. (Schaber et al., ibid; Reiss et. al., ibid; Reiss et al., PNAS, 88:732-736 (1991)). Such inhibitors may inhibit protein farnesylation while serving as alternate substrates for the farnesyl-protein transferase enzyme, or may be purely competitive inhibitors (U.S. Pat. No. 5,141,851; N. E. Kohl et al., Science, 260:1934-1937 (1993); Graham, et al., J. Med. Chem., 37, 725 (1994)). In general, deletion of the thiol from a CAAX derivative has been shown to dramatically reduce the inhibitory potency of the compound. However, the thiol group potentially places limitations on the therapeutic application of FPTase inhibitors with respect to pharmacokinetics, pharmacodynamics and toxicity.
It has recently been reported that prenyl-protein transferase inhibitors are inhibitors of proliferation of vascular smooth muscle cells and are therefore useful in the prevention and therapy of arteriosclerosis and diabetic disturbance of blood vessels (JP H7-112930).
It has recently been disclosed that certain tricyclic compounds which optionally incorporate a piperidine moiety are inhibitors of FPTase (WO 95/10514, WO 95/10515 and WO 95/10516). Imidazole-containing inhibitors of farnesyl protein transferase have also been disclosed (WO 95/09001 and EP 0 675 112 A1).
It is, therefore, an object of this invention to develop peptidomimetic compounds that will inhibit prenyl-protein transferase and thus, the post-translational prenylation of proteins. It is a further object of this invention to develop chemotherapeutic compositions containing the compounds of this invention and methods for producing the compounds of this invention.